A method, illustrated in FIG. 1, for manufacturing a composite structure comprising, from its rear face to its front face, a supporting substrate 1, a covering layer 2, at least one dielectric layer 3 and a useful layer 4, and known from the prior art (see, for example, the document EP 1780794), comprises the following steps:                a) providing a donor substrate 5 and the supporting substrate 1;        b) forming at least the dielectric layer 3 comprising:                    a first surface in contact with the donor substrate 5,            a second surface opposite to the first surface,            a peripheral surface connecting the first and second surfaces together;                        the dielectric layer 3 has a contour Cz;        c) forming the covering layer 2 arranged so as to cover the second surface of the dielectric layer 3;        d) forming a weakened zone 6 in the donor substrate 5 delimiting the useful layer 4 in contact with the first surface of the dielectric layer 3;        e) assembling the supporting substrate 1 and the donor substrate 5, so that the supporting substrate 1 and the covering layer 2 are in contact along a contact surface 7 having a contour Cs;        f) breaking the donor substrate 5 along the weakened zone 6.        
Hereinafter, the assembly formed by the useful layer 4, the dielectric layer 3 and the covering layer 2 will be designated by the term stack of layers 8.
At the end of step f), the stack of layers 8 is transferred onto the supporting substrate 1 in order to form the composite structure.
As illustrated in FIG. 2, the composite structure has a peripheral ring 9.
This peripheral ring 9 is situated in a peripheral zone of the supporting substrate 1 and, in which, in the absence of sufficient adhesion between the supporting substrate 1 and the donor substrate 5, the transfer of the stack of layers 8 does not take place.
Thus, a step is observed at the limit separating the peripheral ring 9 from the stack of transferred layers 8.
Moreover, the flanks of the covering layer 2 and dielectric layer 3 are exposed at the step and, therefore, not protected from any chemical attacks.
Consequently, the chemical attack may generate particles via delamination of the useful layer 4.
When only the dielectric layer 3 or the covering layer 2 is formed, for example, the dielectric layer 3, this step is also observed. A creep of the useful layer 4 is generally executed so as to cover or encapsulate the dielectric layer 3 at the step.
However, the applicant has found that, when there is a covering layer 2 and at least one dielectric layer 3, the step of breaking at the edge of the substrate is atypical. FIG. 3 thus shows the steps obtained on such a substrate after the breaking step f).
When there are several intermediate layers present, the breaking step does not lead to a single step but, on the contrary, to several steps. It appears in fact that the break propagates at the periphery of the substrate, not along the weakened zone but at the interface between the dielectric layer 3 and the covering layer 2.
The main drawback of this manufacturing method is, therefore, that it leads to an atypical break at the edge of the substrate.
This is, in particular, the case when the composite structure comprises, from its rear face toward its front face, a silicon substrate, a silicon dioxide layer, a silicon nitride layer, a silicon dioxide layer and a silicon layer.
Moreover, the presence of several steps makes it impossible for the creep to take place by heat treatment of the useful layer 4 so as to protect the covering layer 2 and the dielectric layer 3 at the step.
This is because, during heat treatment, dewetting of the useful layer 4 rather than creep is observed.
One aim of the disclosure is, therefore, to propose a method for manufacturing a composite substrate that makes it possible to execute a step of creep of the useful layer 4 so as to cover the exposed surface of the covering layer 2 and of the dielectric layer 3 at the step.